Lubricating oils have been used in internal combustion engines and automatic transmissions so as to facilitate the smooth operation thereof. Particularly, lubricating oils for internal combustion engines (engine oils) have been required to possess high characteristic performances due to the fact that recent developed engines have been improved in performances, increased in power output and used under more sever operation conditions and further to be improved in long-drain properties enabling the prolongation of oil drain intervals from the view point of recent environmental issues. Therefore, in order to fulfill such requirements, conventional engine oils are blended with various additives such as anti-wear agents, metallic detergents, ashless dispersants, and anti-oxidants so as to improve their characteristic performances. For example, attempts have been done wherein lubricating base oils are highly refined, synthetic oils such as poly α-olefin and polyolester are used, and anti-oxidation additives such as zinc dialkyldithiophosphates (ZDTP), phenol- and amine-based anti-oxidants and organic molybdenum compounds are optimized. Among these anti-oxidants, sulfur-containing additives such as ZDTP have been used as anti-oxidants and anti-wear agents which are indispensable additives for current lubricating oils, particularly for internal combustion engines.
Examples of ZDTP-free lubricating oils are known to include those containing zinc dialkyldithiocarbamates or sulfur-based additives, such as those disclosed in Japanese Patent Laid-Open Publication Nos. 52-704, 62-253691, 63-304095 and 6-41568 and Japanese Translations of PCT International Publication Nos. 62-501572, 62-501917 and 1-500912. The lubricating oils described in these publications contain sulfur in a large amount similarly to those containing ZDTP.
However, it now becomes apparent that there is a limit to further prolongation of life of a lubricating oil because these additives containing sulfur such as ZDTP possess anti-oxidation properties but release sulfuric acid in the process of their oxidation or thermal decomposition, which sulfuric acid accelerates the oxidation deterioration of the lubricating oil. A lubricating oil composition containing metallic detergents or ashless dispersants was likely to accelerate the decrement (deterioration) of the base number which is an index of acid neutralization properties.
Therefore, it is now necessary to reassess drastically the blend of additives with a central focus on anti-wear agents so as to produce a long-drain type lubricating oil with extremely excellent oxidation stability.
Furthermore, the lubricating oils with low sulfur, phosphorus and ash contents have been strongly demanded as a result of necessity to alleviate harmful influences on exhaust-gas purifying catalysts such as ternary catalysts, oxidation catalysts and NOx adsorbers, or exhaust-gas after-treatment devices such as diesel particulate filters (DPF), which are equipped in internal combustion engines in conformity with recent environment-related issues.
In view of the foregoing circumstances, a first object of the present invention is to provide a lubricating oil composition with excellent long-drain properties such as oxidation stability and base number retention properties and high-temperature detergency, by optimizing an anti-wear agent to be used.
The inventor of the present invention has already found that a lubricating oil composition containing less or no zinc dialkyldithiophosphate having been used conventionally but containing a specific phosphorus-containing compound was able to exhibit extremely excellent long-drain properties (oxidation stability, base number retention properties and thermal stability) while maintaining anti-wear properties equally to a composition containing zinc dialkyldithiophosphate and filed a patent application for such a lubricating oil composition, as disclosed in Japanese Patent Laid-Open Publication No. 2002-294271.
It was confirmed that a lubricating oil composition containing such a specific phosphorus compound was able to exhibit anti-wear properties evaluated by a valve train wear test for domestically produced automobile engines as represented by JASO M328-95 comparably to a composition containing zinc dithiophosphate, due to the optimized blend with other additives. However, a lubricating oil is required to possess extreme pressure properties and anti-wear properties more excellent than ever so as to be used in a special engine operating under more severe conditions or used under particular circumstances where more excellent extreme pressure properties and anti-wear properties are required; or required to fulfill a requirement that the phosphorus content is decreased to 0.08 percent by mass or less to meet the suitableness for an exhaust-gas purifying catalyst in the forthcoming ILSAC GF-4 standard or another requirement of low phosphorus content that the phosphorus content is decreased to 0.05 percent by mass or less to be sought in ILSAC GF-5 standard which is a plan under consideration. However, it is difficult to decrease the phosphorus content of a lubricating oil with a sulfur-free phosphorus compound because extreme pressure properties and anti-wear properties are deteriorated when the phosphorus content is simply decreased. Alternatively, the use of a sulfur-containing compound increases the sulfur content of a lubricating oil which thus exerts harmful influence on an exhaust-gas after-treatment device, i.e., fails to dissolve the problems that an exhaust-gas purifying catalyst such as a ternary catalyst, an oxidation catalyst and a NOx adsorber and DPF or an exhaust-gas treatment system which is the combination of DPF with the exhaust-gas purifying catalyst, particularly the oxidation catalyst or NOx adsorber undergo to catalyst poisoning and/or clogging of DPF caused by the increased sulfur. Furthermore, the lubricating oil will be extremely deteriorated in oxidation stability, base number retention properties and detergency.
Therefore, it has been very difficult to produce a lubricating oil which can achieve the decrease of phosphorus and sulfur or additionally ash content with maintaining both excellent long-drain properties and anti-wear properties. Such a lubricating oil has been demanded.
In view of the foregoing circumstances, a second object of the present invention is to provide a lubricating oil composition which is further improved in anti-wear properties with maintaining long-drain properties in a higher level, particularly suitable for a low phosphorus and sulfur type internal combustion engine.